A commission appointed by the US Department of Energy is studying different options for dealing with spent fuel from nuclear power plants. New Scientistweighs the pros and cons of each.

Salt

Beds of salt up to 1 kilometre thick lie within 1 or 2 kilometres of the surface across much of the US. They were deposited hundreds of millions of years ago by evaporating seas.

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Room-temperature radioactive waste from the production of nuclear weapons has already been stored in one salt deposit – in the Waste Isolation Pilot Plant near Carlsbad, New Mexico, since 1999. Now the US and Germany are considering salt for future waste repositories.

Pros: Salt can flow like slow-moving Silly Putty - so any fissures seal themselves. Buried waste would relatively quickly become encapsulated by the salt, which would close in around it due to pressure from surrounding rock.

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Cons: The still-hot waste could cause small amounts of water trapped in the salt around it to move, carrying with it radioactive materials.

(Image: New Scientist)

Shale

'Shale' refers to a wide range of clay-based sedimentary rocks, ranging from soft mudstones to harder, more compact rock types such as argillite. Different types of shale were deposited in different eras over the last 550 million years (pictured), when fine particles suspended in the waters of river deltas or off the continental shelf settled and cemented into rock.

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France plans to open the first underground nuclear waste repository in shale in 2025.

Pros: Shale's properties mean it could trap radioactive material that passes through the rock. Soft shale is unlikely to fracture and is self-sealing, like salt. Shale deposits are found throughout much of the US, including areas of low seismic activity.

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Cons: Hard, compacted shale is prone to fracturing and doesn't self-seal. Shale deposits in the US have not been studied in enough detail to determine their suitability for storing nuclear waste.

(Image: S Gonzales and KS Johnson, 1984, Shale and other argillaceous strata in the United States/Oak Ridge National Laboratory)

Granite

Granite is known as bedrock for reason. It's a hard, solid rock that is incredibly resistant to wear. Near-surface deposits of the rock formed hundreds of millions of years ago from cooling magma.

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In the 1980s, the US temporarily stored spent nuclear fuel in an underground research laboratory surrounded by granite in Nevada. Finland and Sweden are pursuing nuclear waste repositories in granite, the first of which is scheduled to open in Finland in 2020. China, Japan, Switzerland, and the UK are also giving the hard rock a closer look.

Pros: Strong, stable rock that has a long history of being mined.

Cons: Granite is brittle and fractures easily, creating fissures through which nuclear waste could migrate into surrounding environments and possibly freshwater aquifers. Safe storage would require man-made barriers made of metals such as copper or stainless steel, but these could potentially fail while the waste was still radioactive.

Deep borehole disposal

This concept calls for burying the waste deeper than the other proposals, in granite or other hard rocks. The approach relies on new drilling technology that makes it possible to bore a 0.5-metre-wide hole as far as 5 kilometres below the surface. Canisters of spent nuclear fuel would be lowered into the bottom 2 kilometres of each borehole and then covered with a 3-kilometre-thick seal of clay, asphalt and concrete.

Pros: Deep boreholes offer all the strengths of granite, plus they are far below any freshwater aquifers. If any radioactive material seeped into the surrounding environment, it would remain trapped in dense, highly saline water.

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Cons: The concept is untested.

(Image: New Scientist)

Reprocessing

Another option is to reprocess or recycle as much of the spent fuel as possible for further use. France, the UK, Russia, India and Japan currently do this. The US does not, but is considering the possibility as part of the current Department of Energy review.

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Pros: Reprocessing recovers unused uranium and plutonium from spent fuel for reuse, thereby increasing the energy output from the original fuel by approximately 25 per cent. The process also reduces the volume of material to be disposed of as high-level waste to about 20 per cent of what it would otherwise be, according to the World Nuclear Association.

Cons:Plutonium that has been separated from used fuel can be used to make nuclear weapons, raising fears that it could be stolen.

(Image: Steve Allen/Brand X/Getty)

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